The spec says this is invalid, but it matches giflib's behavior.

Fixes #9856 (together with https://go-review.googlesource.com/11661).

Change-Id: I05701f62a9e5e724a2d85c6b87ae4111e537146b
Reviewed-on: https://go-review.googlesource.com/11663Reviewed-by: Rob Pike <r@golang.org>